RhIII-Catalyzed [4 + 1] Annulations of 2-Hydroxy- and 2

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RhIII-Catalyzed [4 + 1] Annulations of 2‑Hydroxy- and 2‑Aminobenzaldehydes with Allenes: A Simple Method toward 3‑Coumaranones and 3-Indolinones Ramajayam Kuppusamy, Parthasarathy Gandeepan, and Chien-Hong Cheng* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: A novel method for the regio- and stereoselective synthesis of substituted 3-coumaranones from salicylaldehydes and allenes using a rhodium(III) catalyst has been developed. This procedure gives access to new 2-vinyl-substituted 3-coumaranone compounds. The method involves a RhIII-catalyzed aldehyde C−H activation and annulation reactions. Moreover, this RhIII-catalyzed [4 + 1] annulation reaction has been applied to 2-aminobenzaldehydes to afford 2,2-disubstituted 3-indolinones.

and to the best of our knowledge, there is currently no direct method available for their synthesis in the literature. Transition-metal-catalyzed C−H activation reactions have emerged as a promising avenue in organic synthesis.6 In particular, coordination-assisted C−H bond cleavage followed by coupling with π-components is an attractive strategy for the synthesis of carbocyclic and heterocyclic compounds.7 In addition to aromatic C(sp2)−H and aliphatic C(sp3)−H bond functionalization, aldehyde C(sp2)−H activation has also become increasingly popular in recent years.8 Owing to our interest in the area of transition-metal-catalyzed C−H functionalization,9 we developed a method based on an ohydroxyl-group-assisted aldehyde C(sp2)−H cleavage followed by coupling with allenes to afford 2-vinyl-substituted 3coumaranones. Treatment of salicylaldehyde (1a) and buta-2,3-dien-1ylbenzene (2a) in the presence of 2.0 mol % of [RhCl2Cp*]2, and 2.1 equiv of Cu(OAc)2 in N,N-dimethylformamide (DMF) (2 mL) at 90 °C for 15 h gave (E)-2-methyl-2-styrylbenzofuran-3(2H)-one (3aa) in 88% isolated yield. The product was characterized using 1H and 13C NMR, along with high resolution-mass spectrometry (HR-MS). The choice of solvent and oxidant play a crucial role in the reaction. Among the various tested solvents, the formation of product 3aa was less effective in MeOH, CH3CN, THF, and (CH3)2CO. In addition, the catalytic reaction was ineffective when other oxidants (AgOAc, Ag2O, and O2) were used instead of Cu(OAc)2. Nonetheless, using a catalytic amount of Cu(OAc)2 (0.2 equiv) and O2 in the reaction gave 11% product yield. The controlled experiment revealed that no product was formed in the absence of [RhCl2Cp*]2 (see the Supporting Information for detailed optimization studies). After obtaining the optimal reaction conditions, we examined the reaction of various substituted salicylaldehydes 1b−p with

3-Coumaranones (benzofuran-3(2H)-one) are an important structural motif found in many natural and bioactive compounds (Figure 1).1,2 They are also an important building

Figure 1. Examples of natural products containing the 3-coumaranone motif.

block in the synthesis of heterocycles and molecules with medicinal properties.3 Classical methods to synthesize 3coumaranone derivatives mainly involve the AlCl3-mediated cyclization of 2-phenoxyacetyl chlorides or the base-mediated Dieckmann reaction of ethyl 2-(2-formylphenoxy)acetates.4 However, these methods are not widely used due to the limited availability of starting compounds, and harsh acid or base reaction conditions required. Therefore, the development of a facile synthetic method to synthesize 3-coumaranone derivatives with a broad substrate scope is highly desired.5 Herein, we report a convenient method for the synthesis of 3coumaranones from readily available salicylaldehydes and allenes through rhodium(III)-catalyzed aldehyde C−H activation and [4 + 1] annulation reactions. Furthermore, our method gives access to 2-vinyl-substituted 3-coumaranones, © XXXX American Chemical Society

Received: June 25, 2015

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DOI: 10.1021/acs.orglett.5b01825 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters 2a (Scheme 1). Reaction of 3-Me-, 3-OMe-, and 3-Brsubstituted salicylaldehydes 1b−d with 2a afforded the

Scheme 2. Scope of Allenes in the Synthesis of Substituted 3Coumaranonesa,b

Scheme 1. Scope of Salicylaldehydes in the Synthesis of Substituted 3-Coumaranonesa,b

a

Reaction conditions: salicylaldehyde 1 (0.40 mmol), allene 2a (0.60 mmol), [RhCl2Cp*]2 (0.008 mmol), and Cu(OAc)2 (0.840 mmol) in DMF (2 mL) at 90 °C for 15 h. bIsolated yields.

Meanwhile, the unsymmetrical internal allene 2l gave two regioisomeric products 3al + 3al′ in 66% combined yield. The reaction of substituted 2-aminobenzaldehydes with allenes using the RhIII catalyst was also examined.8k,10 Treatment of 2-(tosylamino)benzaldehyde (4a) with 2a, under the reaction conditions aforementioned, gave (E)-2methyl-2-styryl-1-tosylindolin-3-one (5aa) in a meager 7% yield. However, increasing the reaction temperature to 130 °C provided a maximum yield of 45%. After performing detailed optimization studies, we found that the reaction of 4a (0.18 mmol), 2a (0.36 mmol), [RhCp*Cl2]2 (0.0036 mmol), and Cu(OAc)2 (0.36 mmol) in CH3NO2 at 130 °C for 15 h afforded 5aa in 82% isolated yield (see the Supporting Information for details). It is worth mentioning that the indolin-3-one skeleton is an important structural moiety found in many natural products.11 To study the scope of the RhIII-catalyzed formation of 3indolinone derivatives, we examined the reaction of different substituted 2-aminobenzaldehydes with allenes under the optimized reaction conditions (Scheme 3). Thus, the reaction of 4a with o-, m-, and p-methylbenzylallenes (2b−d) afforded the desired [4 + 1] annulation products 5ab−ad in good yields. Similarly, allenes 2e and 2f reacted with 4a to give products 5ae and 5af in 63% and 56% yields, respectively. Bromo-substituted 2-aminobenzaldehyde 4b was also effectively transformed to the corresponding indolinone 5ba in 60% yield. A sterically demanding methyl group at the ortho position (relative to the aldehyde moiety) of 4c did not adversely affect the cyclization reaction, giving 5ca in 68% yield. Moreover, we also examined the effect of different amino protecting groups in the RhIIIcatalyzed [4 + 1] annulation reaction. The reaction of 3-OMe

a

Reaction conditions: salicylaldehyde 1 (0.40 mmol), allene 2a (0.60 mmol), [RhCl2Cp*]2 (0.008 mmol), and Cu(OAc)2 (0.840 mmol) in DMF (2 mL) at 90 °C for 15 h. bIsolated yields.

products 3ba−da in 84%, 90%, and 87% yields, respectively. Similarly, 4-substituted salicylaldehydes (1e−g) gave the desired products 3ea−ga in excellent yields. Next, we tested various 5-substituted salicylaldehydes (1h−m) under the same reaction conditions to afford the desired products (3ha−ma) in 85−93% yields. The reaction of 3,5-disubstituted salicylaldehydes (1n and 1o) also provided the desired products 3na and 3oa in high yields. Using the optimized reaction conditions, 2hydroxy-1-naphthaldehyde (1p) and 2a reacted to give the corresponding [4 + 1] annulation product 3pa in 93% yield. Next, we investigated the scope of the allenes for the rhodium(III)-catalyzed [4 + 1] annulation reaction (Scheme 2). 2-Me-, 3-Me-, and 4-Me-substituted benzyl allenes (2b−d) reacted with 1a to give the desired cycloaddition products 3ab−ad in 82−89% yields. Similarly, 5-phenyl-1,2-pentadiene (2e) and 6-phenyl-1,2-hexadiene (2f) gave the expected products 3ae and 3af in 78% and 73% yields, respectively. 1Naphthyl-substituted allene 2g afforded the product 3ag in 83% yield, whereas alkyl allene 2h gave the product 3ah in 70% yield under similar reaction conditions. Penta-3,4-dien-1-ol (2i) reacted with 1a to give 2-allyl alcohol substituted 3coumaranone (3ai) in 68% yield. The catalytic reaction also proceeded well with 1-methoxy-4-(penta-3,4-dien-2-yl)benzene (2j) to afford the product 3aj in 82% yield. Cyclic internal allene 2k underwent [4 + 1] cycloaddition with 1a to furnish the spiro 3-coumaranone derivative 3ak in good yield (60%). B

DOI: 10.1021/acs.orglett.5b01825 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 3. Scope of the RhIII-Catalyzed [4 + 1] Annulation of 2-Aminobenzaldehydes and Allenesa,b

rivative 6 in 87% yield. Similarly, 2-methyl-2-styryl-2,3dihydrobenzofuran-3-ol (7) was obtained by the reduction of 3aa with NaBH4. A selective reduction of the alkenyl C−C double bond of 3aa using Pd/C and H2 generated 2-methyl-2phenethylbenzofuran-3(2H)-one (8) in 85% yield. On the basis of our experimental results and known literature, a plausible catalytic cycle of the [4 + 1] annulation reaction of salicylaldehydes and allenes is presented in Scheme 5 (using 1a and 2a as examples).8,12 The formation of the Scheme 5. Proposed Reaction Mechanism

a

Reaction conditions: 2-aminobenzaldehyde 4 (0.18 mmol), allene 2 (0.36 mmol), [RhCl2Cp*]2 (0.0036 mmol), and Cu(OAc)2 (0.36 mmol) in CH3NO2 (2 mL) at 130 °C for 15 h. bIsolated yields.

[RhIII] monomer from the [RhIII] dimer precatalyst initiates the catalytic cycle. Coordination of the OH group on the salicylaldehyde to the RhIII complex, followed by an aldehyde C(sp2)−H bond cleavage, formed the five-membered rhodacycle I. Subsequently, coordination of the allene 2a to the rhodium(III) center of I, followed by a regioselective intramolecular insertion of the allene into the carbon−rhodium bond of II, provides the rhodium−π-allyl complex III. A consecutive intramolecular insertion of the oxygen−rhodium bond into the π-allyl complex affords the intermediate IV. The final product 3aa is delivered by a β-hydride elimination process from the intermediate IV. The resulting Rh−H complex undergoes reductive elimination to give [Cp*RhI], which is then oxidized by Cu(OAc)2 to regenerate an active [Cp*RhIII] catalytic species. In summary, we have developed a novel method for the RhIIIcatalyzed synthesis of 2,2-disubstituted 3-coumaranones from substituted salicylaldehydes and allenes. The reaction leads to the formation of 2-vinyl-substituted 3-coumaranones under mild reaction conditions with a broad substrate scope. A possible mechanism involves the phenolic hydroxyl group directed aldehyde C(sp2)−H activation and annulation cycle. Our method has also been applied to 2-aminobenzaldehydes to give 2-vinyl-substituted 3-indolinones. A detailed mechanistic study and the application of this synthetic strategy to asymmetric systems are now in progress.

and 4-Br benzenesulfonyl-protected substrates 4e and 4f with 2a afforded the desired products 5ea−fa in good yields. Similarly, 2-naphthylsulfonyl-protected aminoaldehyde 4g coupled with 2a to give 5ga in 67% yield. However, the treatment of N-(2-formylphenyl)methanesulfonamide (4h) with 2a gave 5ha in only 40% yield. Unfortunately, acetyland pivaloyl-protected aminobenzaldehydes failed to undergo the [4 + 1] cyclization reaction. Since the synthesized 3-coumaranone and 3-indolinone derivatives contain multiple functional groups, they can be used in diversity-oriented synthesis (DOS) to generate useful structurally diverse compounds. As demonstrated in Scheme 4, the Wittig reaction between the keto group of 3aa and methyltriphenylphosphonium afforded dihydrobenzofuran deScheme 4. Diversity-Oriented Synthesis of Compounds 6−8 from 3aa

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Organic Letters

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b01825. General experimental procedures, characterization details, 1H and 13NMR spectra of new compounds, and Xray data (PDF) X-ray data for compound 3ga (CIF) X-ray data for compound 5ea (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Home page: http://mx. nthu.edu.tw/~chcheng/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of the Republic of China (MOST 103-2633-M-007-001) for support of this research.

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DOI: 10.1021/acs.orglett.5b01825 Org. Lett. XXXX, XXX, XXX−XXX